bhyve: Do not enable PCI BAR decoding if a boot ROM is present

Let the boot ROM handle BAR initialization. This fixes a problem where
u-boot's BAR remapping conflicts with some limitations in bhyve. See
https://lists.freebsd.org/archives/freebsd-virtualization/2024-April/002103.html
for a description of what goes wrong.

The old behaviour can be restored by setting the pci.enable_bars
configuration variable.

Reviewed by: corvink, jhb
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D45049